PROJECT SUMMARY/ABSTRACT Alzheimer?s disease (AD) is a neurodegenerative disease that results in amyloid ? plaque deposition, neurofibrillary tangle (NFT) formation, and life-altering cognitive defects. Many human genetic AD risk factors, including APOE, CLU, and TREM2, modulate neuroinflammation and/or the function of microglia, the central nervous system (CNS) resident innate immune cell. Deletion of all peripheral adaptive immune cells (i.e. CD4+ T cells, CD8+ T cells and B cells) or T cell depletion (i.e. CD4+T cells, CD8+T cells) increased cognitive abilities in amyloidosis and tauopathy models, respectively, and were associated with altered microglial function. Interestingly, aging, another major AD risk factor, is associated with inflammation and we have found that increases in CNS CD8+ T cells found with aging is further exacerbated by amyloidosis. Additionally, enhanced CNS CD8+ T cells numbers are also found during tauopathy. We hypothesize that CD8+ T cells impact cognition and CNS sequelae during tauopathy and age-associated amyloidosis. We predict that CD8+ T cells alter microglia function and transcriptomes as a mechanism for disease modulation. To address this hypothesis, we will utilize aged wild type (WT) control mice, aged APPNL-F/NL-F mice, which express human amyloid precursor protein (APP) or our AAV1 model of tauopathy, respectively, on a normal WT or CD8-/- background, to eliminate CD8+ T cells. We will also conduct similar studies on an OT-I background which contains CD8+ T cells that are not stimulated through their T cell receptor (TCR). We will assess multiple parameters including behavioral/ cognitive performance (using open field assay, elevated plus maze, contextual fear conditioning and morris water maze), immunofluorescence/immunohistology examining plaque deposition, tau phosphorylation, CD8+ T cell localization and microglial/ astrocyte reactivity, flow cytometric analysis of CNS CD8+ T cell function, western blot analysis for total and phosphorylated tau, ex vivo microglial cultures and RT-qPCR to examine cortical and hippocampal gene expression of proinflammatory and anti- inflammatory factors. Furthermore, we will be using single cell transcriptomics to examine the entire RNA transcriptome on a per cell basis to assess CNS CD8+T cell transcriptomes and the impact of CD8+T cells on microglial transcriptomes and subpopulations during normal aging, age-associated amyloidosis or tauopathy. Our proposed work will be the first to define the transcriptomes of CNS CD8+ T cells in the context of age- associated amyloidosis or tauopathy. Additionally, we will address for the first time how CD8+T cells impact behavior/cognition, neuroinflammation, gliosis, microglial transcriptomes and pathology during age-associated amyloidosis or tauopathy. This work will provide highly novel insights into how peripheral and central immunity interact during aging and disease, and could provide the impetus to examine new therapeutic measures for management/alleviation of AD associated CNS sequelae.